According to https://bipartisanpolicy.org/explainer/25th-amendment-frequently-asked-questions/:

Section 2 of the 25th Amendment requires that if the office of the vice president becomes vacant, the president nominates a new vice president who must then be confirmed by a majority vote of both the House of Representatives and the Senate. This provision ensures that the office of the vice president is never left unoccupied for long.

But if the House or Senate delay or refuse to confirm the VP selection, it effectively means the Speaker of the House is next in line. Which means if anything was to happen to President Harris, we would be dealing with President Mike Johnson.

And before you think surely, nothing would delay that approval, recall that the Senate Majority Leader held up a Supreme Court vacancy from coming up for a vote for nearly a year until the President of his own party was in power.